1. Field of the Invention
The present invention relates to an improved optical system for converging divergent rays of light emitted from a light source onto an optical disc to optically record data and for reading (reproducing) the optical data recorded on the optical disc.
2. Description of Related Art
In a known optical disc apparatus, a semiconductor laser which emits coherent light is usually used as a light source. The bundle of rays emitted from the semiconductor laser is divergent light having an elliptical shape in cross section. The divergent light is collimated by a collimating lens and is converged onto the optical disc through an objective lens.
Both the beam that is converged onto tile optical disc to record data on the optical disc, and the beam that is converged onto the optical disc to erase recorded data from the optical disc, must have high energy. It is therefore necessary to effectively utilize the amount of light emitted from the semiconductor laser. To increase the light utilization efficiency, a collimating lens having a large numerical aperture (short focal length) is used. However, the width of the bundle of rays on the minor diameter side, transmitted through such a collimating lens having a large numerical aperture, is smaller than the aperture of the objective lens, making it impossible to effectively converge all rays onto the optical disc. To solve this problem, usually one or two anamorphic prisms are employed to change the ellipticity of the collimated rays to thereby increase the width of the light beam on the minor diameter side. As a result, the adjusted beam meets (i.e., is equal to) the aperture of the objective lens to obtain a desired beam spot at a predetermined pit (position) on the optical disc. If the divergence angle of the beam emitted from the semiconductor laser is extremely small, it is very difficult to satisfy the requirements of the aperture of the objective lens, whereas if the divergence angle is extremely large, the light utilization efficiency is decreased. Under these circumstances, the semiconductor laser to be used is determined in accordance with the divergence angle thereof.
The anamorphic prism functions to shape the elliptical laser beam into a substantially circular beam. If two anamorphic prisms are used, they are usually made of the same glass.
However, in view of irregularities in the divergence angle between the products (semiconductor lasers) which might possibly occur as a result of manufacturing errors, etc., it is nearly impossible to obtain a desired beam spot quality in all such lasers. Therefore, it is likely that some lasers will have poor quality (e.g., decreased precision) in the recording and reproduction of data or signals.
Furthermore, the oscillation wavelength of the semiconductor laser slightly fluctuates depending on the output thereof, ambient temperature, etc. Accordingly, if there is a change in the oscillation wavelength of the semiconductor laser in the conventional optical disc apparatus, the divergence angle of the beam emitted from the anamorphic prism(s) changes, so that the beam spot is deviated from a predetermined pit of the optical disc, resulting in a decreased precision of recorded and reproduced signals.